It is theory that decides what can be observed.

Even for practical purposes theory generally turns out the most important thing in the end.

Let us work without theorizing, 'Tis the only way to make life endurable.

A conception not reducible to the small change of daily experience is like a currency not exchangeable for articles of consumption; it is not a symbol, but a fraud.

Delight at having understood a very abstract and obscure system leads most people to believe in the truth of what it demonstrates.

Some theories are good for nothing except to be argued about.

Socrates thought and so do I that the wisest theory about the gods is no theory at all.

In order to shake a hypothesis, it is sometimes not necessary to do anything more than push it as far as it will go.

To insure the adoration of a theorem for any length of time, faith is not enough, a police force is needed as well.

A theory is the more impressive the greater is the simplicity of its premises, the more different are the kinds of things it relates and the more extended the range of its applicability.